The retirement of Justice Anthony Kennedy, announced Wednesday in a letter hand-delivered to President Trump, could bring about sweeping changes to U.S. environmental law, endangering the federal government’s authority to fight climate change and care for the natural world.

With Kennedy gone, a more conservative Supreme Court could overhaul key aspects of the Clean Air Act, the Clean Water Act, and the Endangered Species Act, legal scholars say. And any new justice selected by President Trump would likely seek to weaken the Environmental Protection Agency, curtail its ability to fight global warming, and weaken its protections over wetlands.

The reason has to do with simple math. As on many other issues, Kennedy has functioned as the court’s swing vote on the environment, occasionally joining with the court’s four more liberal justices to preserve some aspect of green law.

“He’s been on the court just over 30 years, and he’s been in the majority in every single environmental case but one. You don’t win without Kennedy,” said Richard Lazarus, a law professor at Harvard who has argued 14 cases in front of the Supreme Court.

“I think more than the other more conservative justices, Kennedy seemed open to embracing the idea that tough national laws were necessary to address some types of problems,” he told me. “He was concerned about private-property rights and the marketplace, but open to the necessity of tough environmental laws.”

Other legal scholars and environmental advocates agreed.

“The loss of Kennedy is not good news for environmental regulation,” said Ann Carlson, a law professor at UCLA and the co-director of the Emmett Institute on Climate Change and the Environment.

The nation’s highest court would now “almost certainly” be more hostile to environmental law than it has been since the founding of the EPA in 1970, said Jonathan Z. Cannon, a law professor at the University of Virginia.

“With the departure of Justice Kennedy, this is no time to mince words: We are in for the fight of our lives,” said Trip Van Noppen, the president of the environmental-legal-advocacy group Earthjustice, in a statement. “Trump intends to fill this Supreme Court vacancy with someone who will put corporations, the wealthy, and the powerful above the rest of us. We must do everything in our power to resist this.”

Experts said that Kennedy’s departure could change the outcome of near-term rulings on three different questions: Can the government fight climate change? How broadly can it regulate clean water? And does the Constitution even allow it to protect endangered species and regulate pollution?

Kennedy provided the crucial fifth vote in Massachusetts v. EPA, which is the most important court case in U.S. climate law. In that decision, the Supreme Court said the EPA could regulate greenhouse gases under the Clean Air Act.

Cannon, the University of Virginia law professor whose legal arguments shaped the case, told me that Kennedy’s vote in Massachusetts was “an environmentalist triumph that would not have happened if he had held ranks with other conservatives.”

Had Kennedy not joined with the liberals, then the EPA would likely not have authority over greenhouse gases. With Kennedy gone, this may soon come to pass.

“It’s easy to think about the loss of Kennedy leading to either the repeal of Mass. v. EPA or a serious restriction to the Clean Air Act’s ability to regulate greenhouse gases,” Carlson told me.

Lazarus, the Harvard professor, disagreed that a more conservative court would overturn Massachusetts v. EPA. “I assume [the decision] itself, that greenhouse gases are air pollutants, will hold. That’s a Constitutional law question … I don’t think we’re going to run roughshod over that,” he told me.

But he worried a future court could limit the ability of environmentalists to gain standing. That is, it could seriously restrict the ability of private Americans to sue the federal government for failing to respond to climate change.

Even more likely than these changes to climate law, experts said, is that Kennedy’s successor will curtail the Clean Water Act. Specifically, he or she would make it much easier to—and this is not a joke—drain the swamps.

The Clean Water Act, passed over President Nixon’s veto in 1972, allows the government to regulate the conservation and protection of rivers, streams, and any other “waters of the United States.” It also prevents companies from draining wetlands or dumping pollutants in those waters without a permit.

The only problem: The phrase “the waters of the United States” has never been completely defined. This means that it isn’t totally clear which bodies of water—and especially which wetlands, which tend to touch many different rivers and streams—are subject to EPA conservation and pollution control.

When the Supreme Court last examined that problem, in 2006, the justices ruled in an unusual way: 4-1-4. The four liberal justices wanted to preserve broad protections for wetlands; four conservative justices wanted to overrule it.

Kennedy wound up in the middle. He wrote his own opinion, recommending that wetlands be subject to federal regulation only if they had a “significant nexus” with navigable waters. This argument would have preserved much of the federal government’s ability to regulate wetlands.

A few years later, when the Obama administration tried to define “waters of the United States” once and for all, EPA lawyers looked to Kennedy’s opinion. “The Obama administration’s approach to regulating wetlands basically came straight out of Kennedy’s reasoning in that case,” Carlson told me.

It was a savvy bit of rule-writing: Since Kennedy would likely rule on a case about their rule, why not adopt his legal thinking? The only problem: “Obviously, now, Kennedy will be gone,” Lazarus said.

Had he stayed on the court for another year or two, Kennedy likely would have ruled on this very question. In January, the EPA Administrator Scott Pruitt announced that the Trump administration would suspend the Obama administration’s Kennedy-inspired wetlands rule and replace it with a far weaker policy. States and environmental groups promptly sued Pruitt, setting up a legal fight that has a good chance of reaching the high court.

“I think Kennedy would have struck down a serious attempt to repeal federal jurisdiction [over wetlands],” Carlson told me. “Kennedy had a much more sophisticated view of why the environmental protection of wetlands made sense—and not just for the wetlands themselves. Losing him could wreak havoc.”

Finally, Kennedy’s retirement could allow the high court to rule on a broad Constitutional question about whether the government even has the power to make and enforce environmental policy in the first place.

In October, for instance, the Supreme Court will hear arguments in Weyerhaeuser Company v. United States, which could address whether parts of the Endangered Species Act are unconstitutional. Plaintiffs in that case argue that the federal government doesn’t have Constitutional authority to force land owners to protect the habitats of endangered species.

“That is a hotly contentious issue in the Supreme Court, and it’s a case that one would expect going in would have a good chance of coming out 5-4,” Lazarus said. “So replacing Kennedy with anybody else makes a big difference.”

His absence from the bench could also reopen the Supreme Court to rule on the “takings clause,” which limits the federal government’s ability to take private property “for public use, without just compensation.” During the 1980s, conservative justices pushed for more and more aggressive readings of the takings clause. Decisions increasingly seemed to incline to the idea that the government should compensate companies that it regulated.

But when Kennedy joined the court in 1988, the speed of those rulings slowed. A more conservative court could push the idea again. “If you got a Supreme Court more interested in protecting private-property rights, then you could really get a curtailment of the government’s right to regulate under the Endangered Species Act and the Clean Water Act,” Carlson said.

A more conservative court could also end the legal principle, established during the Reagan administration, that government agencies like the EPA should have a wide latitude in interpreting the laws that affect them. The idea, named “Chevron deference” after a key court case, has undergirded many Democratic policy victories in the past three decades. Now, says Lazarus, it is “certainly at risk.”

He told me he expects deference to be a big issue during the confirmation hearings for Kennedy's replacement.

No matter who Trump picks for the Supreme Court, it is clear that an era of American environmental law—built around the preferences of one man—is coming to an end. Environmental lawyers may no longer work as hard to make their arguments Kennedy-esque: respectful of private property, but comfortable with a muscular government.

“Kennedy shared the right’s concerns about the scope of federal power, but he was reluctant to draw sharp bright lines limiting such power,” said Jonathan Adler, a law professor at Case Western Reserve, in an email. “He was similarly reluctant to endorse a view of standing that would significantly constrain environmental claims. These tendencies meant he was often the swing vote, and his solo opinions often determined the contours of the law.”

I asked Lazarus whether Kennedy’s departure made him more worried to argue another environmental case in front of the court. He waved me off.

“No, not at all, you work with what you’re given,” he said. “It’s the American voters’ job to elect the right people to the White House, and then it’s my job to argue the best case I can in front of the justices that are presented.

“And these sorts of moments,” he added, “underscore the stakes.”

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.

Robinson Meyer is a staff writer at The Atlantic, where he covers climate change and technology.